1. Field of the Invention
The present invention relates to an energy filter that is used in an energy analysis instrument employing a charged-particle beam to achieve high-energy resolution or energy-filtered imaging.
2. Description of the Related Art
In an electron microscope or the like, an omega filter (xcexa9-filter) or the like may be used as an imaging energy filter. It is desired to increase energy dispersion within the energy filter. The energy dispersion provided by an omega filter is generally increased with increasing the distance from the entrance window to the exit window (slit position). However, if this distance is increased, the whole instrument in which the filter is mounted is made bulky. Therefore, limitations are placed on increasing the distance between the entrance window and the exit window when attempting to increase energy dispersion.
Furthermore, the imaging energy filter needs to be mirror-symmetric with respect to the center plane to cancel out second-order aberrations. Therefore, it is difficult to adopt a procedure consisting of increasing the magnification in order to increase the energy dispersion. Consequently, the magnification is generally fixed at 1xc3x97 between the entrance window and the exit window or between the entrance pupil and the exit pupil.
One available method for obtaining a large energy dispersion under the restrictions described above consists of introducing a field acting as a concave lens in the direction of dispersion to increase the deflection action and the focusing action owing to a uniform field without increasing the size. For example, the end surfaces (i.e., the surfaces on the entrance side and on the exit side) of magnetic polepieces for achieving a quadrupole field are tilted. With this method, however, as the tilt angle is increased, the amount of the second aberration increases. Furthermore, the accuracy of simulation made when the filter is designed deteriorates. Accordingly, the end-surface tilt angle is substantially restricted to within approximately 40xc2x0. As a result, the energy dispersion is only about 1 xcexcm at an accelerating voltage of 200 kV where the filter size has practical dimensions.
Another method for increasing energy dispersion while canceling out second-order aberrations is to tilt the mutually opposite pole faces for creating a quadrupole field. FIGS. 6(A) and 6(B) schematically show the configuration of such an omega filter. FIG. 6(A) is a plan view of the omega filter, while FIG. 6(B) is a cross-sectional view taken on line IIIxe2x80x94III of FIG. 6(A). As shown in FIG. 6(B), the mutually opposite surfaces of magnetic polepieces 21 and 21xe2x80x2 are tilted at a given angle along the optical axis O. The surfaces of magnetic polepieces 22 and 22xe2x80x2 (piece 22xe2x80x2 is not shown) are similarly tilted. The magnetic polepieces 21, 21xe2x80x2, 22, and 22xe2x80x2 form parts of a cone. The generatrix of the cone is indicated by 21a and 21b. 
This geometry is effective in enhancing the energy dispersion in the omega filter. Furthermore, the amount of second-order aberration is smaller than where the end surfaces of magnetic polepieces are tilted.
Another energy filter for increasing energy dispersion is described in U.S. Pat. No. 5,449,914. FIG. 7 is a horizontal cross section schematically showing the structure of this energy filter. The energy filter shown in FIG. 7 is equipped with three sector magnets which have bottom magnetic polepieces 31, 32, and 33, respectively. The magnetic polepiece 31 of the first sector magnet has a pole face parallel to the pole face of the top magnetic polepiece and produces a uniform magnetic field. The pole faces of the second and third sector magnets are tilted similarly to the structure shown in FIG. 6(B). Accordingly, the second and third sector magnets produce nonuniform magnetic fields.
Referring still to FIG. 7, the trajectory of a beam incident along the optical axis 34 is bent through a large angle at a radius of rotation of R1 by the first sector magnet. Then, the beam vertically enters the nonuniform magnetic field region produced by the second sector magnet. The beam then passes into the nonuniform magnetic field region produced by the third sector field. The trajectory of the beam is deflected by the magnetic fields developed by the second and third sector magnets. The beam returns into the magnetic field produced by the first sector magnet. The trajectory of the beam is again bent through a large angle by the first sector magnet and reaches the exit slit.
In this structure, the trajectory of the beam incident on the energy filter is bent four times in total and so the length of the trajectory can be made large. Hence, the energy dispersion can be increased. Furthermore, it is possible to bend the trajectory by the first sector magnet such that the beam trajectory from the side of the entrance window and the beam trajectory directed toward the exit slit intersect each other. Consequently, the trajectory length can be increased further.
In this geometry, the two beam trajectories in the magnetic field developed by the first sector magnet need to intersect each other. This complicates the design conditions of the instrument, especially the design conditions of the first sector magnet. That is, this geometry is effective in suppressing increase in size of the energy filter. However, the structure for causing the two beam trajectories to intersect each other is rendered complex.
Accordingly, it is an object of the present invention to provide an energy filter that is relatively simple in structure and capable of providing increased energy dispersion while canceling out second-order aberrations.
An energy filter in accordance with the present invention is equipped with three magnetic field regions through which a charged-particle beam successively passes, and has the following features. The charged-particle beam first goes into and out of the first magnetic field region, where the beam has a radius of rotation of R1. The beam emerging from the first magnetic field region then passes through the second magnetic field region, where the beam has a radius of rotation of R2. The beam going out of the second magnetic field region finally passes through the third magnetic field region, where the beam has a radius of rotation of R1. The three magnetic field regions are so arranged that the optical axis of the beam incident on the first magnetic field region where the beam has the radius of rotation R1 and the optical axis of the beam emerging from the third magnetic field region where the beam has the radius of rotation of R1 are in line. In each of the three magnetic field regions, a nonuniform magnetic field that becomes intenser toward the center of rotation of the beam is produced.
Other objects and features of the invention will appear in the course of the description thereof, which follows.